1. Field of the Invention
The invention relates to a thin film semiconductor device and more particularly to a thin film semiconductor device used in switching element or the like which is provided to each picture element electrode in an active matrix liquid crystal display apparatus, and further to a liquid crystal display apparatus which utilizes the thin film semiconductor device.
2. Description of the Related Art
An active matrix drive system has been adopted to drive a liquid crystal display apparatus. This drive system drives a liquid crystal by providing a switching element and, as required, a signal storage element to each picture element electrode, which are integrated into a single device. In a liquid crystal display apparatus of the drive system mentioned above, a thin film transistor (abbreviated as TFT) or the like is provided as a switching element in the vicinity of each picture element electrode, and a gate electrode and a source electrode cross each other to provide an XY matrix. To the gate electrode, a control signal for switching TFT is applied and to the source electrode, a signal corresponding to information for displaying (hereafter referred to as a display signal) is applied, for driving the thin film transistor and for applying a display signal voltage across to each picture element electrode connected to a drain electrode and a counter electrode opposing thereto. Thereby, the displaying status of the liquid crystal display element is controlled.
FIG. 1 is a plan view of the vicinity of a conventional thin film semiconductor device, and FIG. 2 is a cross-sectional view taken along a line II--II of FIG. 1. Referring to FIG. 2, the construction of the conventional thin film semiconductor device 100 will be explained. In the thin film semiconductor device 100, a thin film transistor includes a gate electrode 101, a semiconductor layer 103 made of amorphous silicon film which is formed over a gate insulating film 102 and the gate electrode 101, a protective insulator film 104 formed over the semiconductor layer, n+ type semiconductor layers 105a and 105b made of phosphorus-doped n+ amorphous silicon film or the like which are formed over both the semiconductor layer 103 and the protective insulator film 104, and a source electrode 106 and a drain electrode 107 formed on the n+ type semiconductor layers 105a and 105b respectively. The thin film transistor is formed on an electrically insulating substrate 109 which is transparent, and the drain electrode 107 is connected to a picture element electrode 108.
The conventional thin film transistor is formed so that the semiconductor layer extends beyond the edge of the gate electrode as illustrated in FIG. 1. As a result, the risk of the leakage which may occur between the gate and the source at the side wall of the gate edge from damage to the gate insulating film during patterning of the n+ type semiconductor layer is reduced. According to the construction of the conventional thin film transistor, it is obvious that problems such as the leakage between the gate and source due to damage of the gate insulating film does not occur.
In a liquid crystal display apparatus of the conventional active matrix drive system, the signals are applied to the gate electrode and the source electrode, for driving the thin film transistor located at each intersection and for controlling the current flowing between the source electrode and the drain electrode. Thereby, optical characteristics, such as rotatory power and transmissivity of the liquid crystal display element, are controlled. The presence of a defect of the ON/OFF switching characteristics in the multitude of thin film transistors provided in the display region will cause deterioration of the display quality such degradation of the contrast and the uniformity. One of the causes of this defect is from an increase in the off current. The off current refers to the leakage current flowing when the thin film transistor is kept off by setting the gate to a low level. The leakage current is caused by the carriers generated in the semiconductor layer of the thin film transistor by thermal excitation and optical excitation, for example.
In an experiment with a transmission type liquid crystal display apparatus formed of the thin film semiconductor 100 illustrated in FIG. 1, it has been confirmed that the light emitted by a light source 110 provided at a side of the substrate 109 opposite to the side where the thin film transistors are formed is irradiated onto the semiconductor layer 103 which is formed to extend beyond the gate electrode 101 on the left and right as illustrated in FIG. 2, for generating the carriers by optical excitation. Thereby an increase of the off current results. Namely, the problem arises from the fact that light irradiates the semiconductor layer 103 because the width L2 of the semiconductor layer 103 is larger than the width L1 of the gate electrode 101.
FIG. 3 is a plan view of another conventional thin film semiconductor device 111 and FIG. 4 is a cross-sectional view taken along a line IV--IV of FIG. 3. Referring to FIG. 3 and FIG. 4, this conventional semiconductor device 111 will be explained. Over an electrically insulating substrate 112 is formed a gate electrode 113 made of chromium, tantalum or the like. Furthermore, a gate insulating film 114 made of silicon nitride, tantalum pentoxide, silicon dioxide or the like is formed for insulating the gate electrode 113 from other electrodes.
On the gate insulating film 114 which covers the gate electrode 113, a semiconductor layer 115 made of amorphous silicon, which may be a channel region of the thin film transistor 111, is formed. Over the semiconductor layer 115 is stacked a protective insulator layer 116 made of silicon nitride or the like which is formed so that the central part remains through after photo-etching or the like. On the part of the semiconductor layer 115 where the protective insulator film 116 has been removed, n+ type semiconductor layers 117a and 117b having electrons as majority carriers formed by phosphorus doping for the purpose of forming a source electrode and a drain electrode. Also n+ type semiconductor layers 117a and 117b cover each edge of the protective insulator film 116, and are arranged to oppose without making contact with each other on the protective insulator film 116. One side of the semiconductor layer 115 is completely covered with both the protective insulator film 116 and the n+ type semiconductor layers 117a and 117b. Thus, the thin film transistors are formed over the substrate 112.
On one of the n+ type semiconductor layers 117a and 117b is formed a source electrode 118 and on the other layer is formed a drain electrode 119. These electrodes are made of chromium, titanium, molybdenum, aluminium or the like. An opposite end to the n+ type semiconductor 117b of the drain electrode 119 is electrically connected to the picture element electrode 120 of the liquid crystal display element. Moreover both the drain electrode 119 and the picture element electrode 120 may be permitted to be integrated with the transparent electrode made of indium tin oxide or the like.
In the following explanation, a direction L and a direction W will be defined as follows. A direction L is (1): an opposed direction of the source electrode and the drain electrode, (2): a direction of the channel of current between the source electrode and the drain electrode, and (3): a direction crossing the gate electrode. The direction L of both the conventional embodiment and an embodiment as described below in the present application satisfies the definitions (1), (2), and (3). Moreover, the direction W is perpendicular to the direction L.
In the conventional structure illustrated in FIG. 3, for the protective insulator film 116, the n+ type semiconductor layers 117a and 117b and the source and drain electrodes 118 and 119 in the direction W perpendicular to the direction where the n+ type semiconductor layer 117a and 117b oppose each other, namely in the lengthwise direction of FIG. 3, the widths W1, W2 and W3 are selected to satisfy the following relation. EQU W1.gtoreq.W2&gt;W3 (1)
Where W1 is the width of the contact region between the semiconductor layer and the n+ type semiconductor layer, W2 is the width of the channel region, and W3 is the width of the source electrode and the drain electrode, respectively.
In the usual operation of the liquid crystal display, light for illumination is irradiated from the opposite side of the substrate 112.
FIG. 5 illustrates a graph of the gate-drain voltage Vgd vs. the drain current Id characteristic. In an initial condition, namely under a dark condition before operating for a prolonged duration under the radiation of light, the gate-drain voltage Vgd vs. the drain current Id characteristic is indicated by the solid line l11 of FIG. 5. Furthermore, the gate-drain voltage Vgd. vs the drain current Id characteristic is measured under a condition where the carriers have been generated in the semiconductor layer 115 by the irradiation of light, and indicates that the characteristic when the TFT is off varies from the solid line l11 of FIG. 5 to as the dashed line L12. The degree of the variation of the off characteristic is dependent on the luminous intensity. If the operating time under the radiation of light were comparatively short enough, the variation between the characteristic l11 and the characteristic l12 would be reversed. Therefore, after the radiation of light pauses, the Vgd-id characteristics measured under a dark condition will revert again to the characteristic shown by the line l11.
On the other hand, after operating for a prolonged duration under the radiation of light, the Vgd-id characteristic shifts to a negative direction of Vgd as represented by the solid line l13 under a dark condition and by the dashed line l14 under the radiation of light. The scale of shift is dependent on the luminous intensity and the operating duration under the radiation of light. Therefore, as the luminous intensity is increased and the operating duration under the radiation of light is prolonged, the shift is larger. The mechanism of this phenomenon will be explained as follows.
When the light is irradiated from another side of the substrate 112, because the gate insulating film 114 is transparent, the light reaches the region in the semiconductor layer 115 where the light is not cut off by the gate electrode 113. The energy of this light will generate the carriers which includes electron-hole pairs in the semiconductor layer 115. In this condition, namely after operating for a prolonged duration under the radiation of light, in the region A, where the electric field intensity in the direction between the source and the drain electrodes is weak, as indicated by the cross hatched area in FIG. 3, the drift time for the carriers (the running time in the channel) will be longer than the drift time in the channel region where the intensity is strong. Therefore, the probability of trapping the carriers into the gate insulating film 114 by the gate-drain voltage Vgd applied to the gate electrode 114 will be high. Accordingly, the threshold voltage Vth will be shifted by the trapped carriers.
In the configuration of the thin film semi-conductor device 111 which is operated for a prolonged duration under the irradiation of light, the switching element will not be completely turned off even when the gate-drain voltage Vgd is so controlled that the switching element is turned off to reserve the carriers accumulated between the display element electrode 120 and the counter electrode. Therefore, the effective voltage applied to the liquid crystal layer will decay. Consequently unevenness may be caused in and the display quality may become uneven or the contrast may become weaker. In other words, as the functional quality of the thin film semiconductor 111 degrades, the reliability of the liquid crystal display element which incorporates the thin film semiconductor 111 is reduced.
FIG. 6 is a plan view of another conventional thin film transistor, where the reference symbols which are the same as the symbols in FIG. 3 represent parts identical or corresponding to the symbols in FIG. 3. The cross-sectional view taken along a line IV--IV in FIG. 6 is the same as the view in FIG. 4. The differences from the thin film semiconductor device 111 of FIG. 3 are the shape of the semiconductor layer 115 and the size of both the protective insulator film 116 and the n+ type semiconductor layers 117a and 117b. In the protective insulator film 116, the n+ type semiconductor layers 117a and 117b and the source and the drain electrodes 118 and 119 in FIG. 6, and the widths W1, W2 and W3 in the same direction as the direction illustrated in FIG. 3 are selected to satisfy the relation: EQU W2.gtoreq.W1&gt;W3 (2)
In other words, W1 is the width of the contact region between the semiconductor layer and the n+ type semiconductor layer, W2 is the width of the channel region, and W3 is the width of both the source electrode and the drain electrode.
In the thin film semiconductor device of the configuration illustrated in FIG. 6, there exists an area A indicated by the cross hatched area where the electric field intensity in the direction between the source and the drain electrodes is weak. Therefore, when the device is operated for a prolonged duration under the irradiation of light, the probability will be high for the carriers to be generated in the semiconductor layer 115 and trapped into the gate insulating film in the area A where the electric field intensity in the direction between the source and the drain electrodes is weak. These trapped carriers cause a shift in the gate-drain voltage Vgd vs. the drain current Id characteristic similar to the case of the thin film semiconductor device 111 of FIG. 4, which results in the degradation of the functional quality of the thin film semiconductor device. This also causes the reliability of the device to be poor.
The models used for the explanation of the undesirable phenomenon in the conventional devices as described above have been conducted by an inventor of the present application and the influence of the light in various shaped TFTs was examined. Therefore, on the basis of these experiments, the embodiments of the present invention propose satisfactory solutions to the problem of the conventional devices.